kenv: allow listing of static kernel environments

kenv: allow listing of static kernel environments

The early environment is typically cleared, so these new options
need the PRESERVE_EARLY_KENV kernel config(8) option. These environments
are reported as missing by kenv(1) if the option is not present in the
running kernel.

(cherry picked from commit db0f26439357b78863e61985acd1e5acf75ce73d)